The use of wireless communication systems is growing with users now numbering well into the millions. One of the popular wireless communications systems is the cellular telephone, having a mobile station (or handset) and a base station. Cellular telephones allow a user to talk over the telephone without having to remain in a fixed location. This allows users to, for example, move freely about the community while talking on the phone.
Cellular telephones may operate under a variety of standards including the code division multiple access (CDMA) cellular telephone communication system as described in TIA/EIA, IS-95, Mobile station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System, published July 1993. CDMA is a technique for spread-spectrum multiple-access digital communications that creates channels through the use of unique code sequences. In CDMA systems, signals can be and are received in the presence of high levels of interference. The practical limit of signal reception depends on the channel conditions, but CDMA reception in the system described in the aforementioned IS-95 Standard can take place in the presence of interference that is 18 dB larger than the signal for a static channel. Typically the system operates with a lower level of interference and dynamic channel conditions.
A CDMA base station communicates with a mobile station with a signal having a basic data rate of 9600 bits/s. The signal is then spread to a transmitted bit rate, or chip rate, of 1.2288 MHz. Spreading applies digital codes to the data bits, which increase the data rate while adding redundancy to the CDMA system. The chips of all the users in that cell are then added to form a composite digital signal. The composite digital signal is then transmitted using a form of quadrature phase shift keying (QPSK) modulation that has been filtered to limit the bandwidth of the signal.
In a code division multiple access (CDMA) spread spectrum communication system, a common frequency band is used for communication with all base stations within that system. If two or more mobile users simultaneously contend for an idle packet-data channel in a system using IS-707, the system will only allow one access to the channel. Mobile users unsuccessful at accessing the channel must repeat the transmission of the data packet until it is accepted by the system. The system users transmitting data packets to mobile users also contend for the downlink by being placed in a queue.
Under the current IS-707 standard, when a system user is unable to access the channel, the system user reattempts connection after a predetermined wait. The length of the wait is defined by the IS-707 standard, and is the same for each system user. After each subsequent unsuccessful attempt to connect to the system, the length of the wait is increased until a maximum value is reached. However, if the system users were denied access to an idle channel because multiple users attempted to simultaneously access the channel, each user will attempt to re-access the channel at the same time, causing further collisions.
What is needed is a system that allows the system users to wait for a free channel to connect to the system while reducing the probability of reconnection collision with other system users.
The present invention distributes the reconnection attempts by the system users over a broad time window. The present invention randomly assigns an initial wait time for each system user from a time window. After the initial wait time is assigned to a system user, any unsuccessful reconnection attempt by that user results in quadrupling the current wait time, until a maximum wait time window is reached. Once the maximum time window is reached, the system user continues to attempt reconnection every cycle of the maximum wait time until connection is maintained. By randomly assigning an initial wait time to each system user, the probability of system users attempting simultaneous reconnection is reduced, thus reducing the likelihood of reconnection collision.
One aspect of the invention is a method of varying the length of time between connection attempts by a mobile station in a wireless communication system. The method comprises initializing a reconnection timer to a set value and waiting for the reconnection timer to time out. After time out, the mobile station attempts to connect to the wireless communication system. If the connection attempt fails, the mobile station resets the timer to a value in a predetermined range. The predetermined range is approximately four times the set value. After the reconnection timer is set, the method may further comprise waiting for the reconnection timer to again time out and attempting to reconnect to the wireless communication system. If the reconnection attempt fails, the mobile station determines if the reconnection timer is at a maximum allowable value. If not at a maximum value, the value of the reconnection timer is increased.
Another aspect of the invention is a mobile station for use in a mobile communication system. The mobile station comprises a reconnection timer and a timer setting circuit. The timer setting circuit sets the reconnection timer to a value within a predetermined range after a failed connection attempt between the mobile station and the mobile communication system. The timer setting circuit also increases the value of the reconnection timer after any subsequent failed connection attempts until a maximum value is reached. The value of the reconnection timer may be increased by quadrupling the previous value.